Is expiation (kaffarah) obligatory for one who swore not to buy from a specific vendor, and then bought from him?

If a mother swore an oath not to buy from a specific vendor, then bought from him, she has broken her oath and an expiation for breaking an oath is incumbent upon her. There is no sin upon her, because breaking an oath is not unlawful unless the oath was to perform an obligation or to abstain from a forbidden act. Breaking an oath becomes obligatory if the oath was to perform a forbidden act or to abstain from an obligation. However, if the oath was concerning a permissible matter, one is given the choice between breaking the oath and fulfilling it, and fulfilling it is more preferable.